HPT and MPI testing. Why?

[tangolima]

The extraction force drops with increased clearance. At zero pressure, a clearance of 0.000", the extraction force is about 100 pounds, at a clearance of 0.005" the extraction force is nearly zero.

If there is pressure inside the chamber, it is pushing the case walls outward, but it is also pushing the case out of the chamber. The force pushing the case out of the chamber subtracts from the extraction force, if this chamber pressure force pushng the case out is greater than the friction force from the case-wall interaction, the extraction force will be negative.

I see it now. Thanks.

It is still hard to understand why 2.5mil clearance still needs 40lbf. The 2 objects are out of contact already. There is air between them. Even with 5mil clearance (more than thickness of printer paper) the force is still non-zero. Where does the resistance come from? Carbon residues filling the clearance?

That aside, the graph gives insight to the difference between steel and brass casing extractions. The force vs. time graph you posted early indicates 20lbf for brass, and there is no peak in the curve. The brass case springs back so much that there is about 3mil clearance to the chamber wall. On the other hand, steel casing has little or no clearance.

-TL



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